1. Technical Field
The present invention relates to a quenchable voltage controlled oscillator (VCO) for a switched band synthesizer and more particularly to a VCO formed, for example, from a heterojunction bipolar transistors (HBT) and a VCO quenching circuit for unquenching a selected VCO and quenching the unselected VCOs; the VCO quenching circuit formed from a p-i-n diode electrically coupled in series with the collector of the HBT, which can be monolithically integrated with the HBT, to provide better performance than known quenchable VCO switchband synthesizer applications.
2. Description of the Prior Art
Heterojunction bipolar transistor (HBT) device technology is known for low phase noise VCO applications due to the low 1/f noise of such devices relative to MESFETs at microwave to millimeter-wave cutoff frequencies. Known HBT VCO devices have superior phase noise performance compared to MESFET VCOs and comparable performance to silicon based VCOs at the C-Ku-band frequencies. However, above such frequencies, HBT technology is preferred for low phase noise VCO applications because of the superior frequency operating capability of HBT based devices compared to the silicon based BJTs and lower device phase noise compared to MESFETs and HEMT technologies as generally described in "Dielectric Resonator Oscillators Using GaAs/(Ga,Al)As Heterjunction Bipolar Transistors," by K. K. Agarwal, 1986 IEEE MTT Symp. Dig., Baltimore, Md., pp. 95-98; and "A Low-Noise Ku-Band A1GaAs/GaAs HBT Oscillator," by N. Hayama, et al., 1988 MTT Symp. Dig., New York, N.Y., pp. 679-682 hereby incorporated by reference. However, for relatively high performance synthesizer applications, phase noise performance over a relatively wide tuning bandwidth provides relatively unacceptable performance. In order to meet the low phase noise and frequency tuning bandwidth requirements, often several VCOs are used which cover several narrower overlapping tuning bandwidths forming a switched band synthesizer. With such a configuration, the narrower VCO tuning bandwidths provide a higher Q in order to achieve lower phase noise.
A relatively important consideration in such a switched band synthesizer application is the use of a VCO quenching circuit which can switch relatively fast and clean, generating little or no switching transient in order to enable the VCOs to phase lock relatively quickly with minimal degradation of the phase noise and pulling performance of the VCO. As setforth in a "10-14 GHz Quenchable MMIC Oscillator", by G. Dietz, et al., 1991 IEEE MTT Symp. Dig., Boston, Mass., pp. 23-26, a known quenchable VCO using MESFET techniques which includes a passive FET switch for providing oscillation quenching is disclosed. This device is illustrated in FIG. 1. Unfortunately, there are several drawbacks to using such a FET switch in the quenching circuit illustrated in FIG. 1. For example, as shown in FIG. 1, the FET switch is disposed between the ground and the source of the FET oscillator which makes it relatively susceptible to frequency pulling due to the active capacitive loading on the source of the oscillating FET of the switch-FET drain. Moreover, as shown, the FET switch is configured in series with the gate-to-source junction, a critical device junction which controls the device transconductance, making the oscillator phase performance very sensitive to the Q of the FET switch. In addition, the use of a noisy FET switch device is relatively more susceptible to VCO carrier modulation than other devices, such as p-i-n diodes thus potentially generating unwanted transients and oscillations. Such spurious effects are relatively more pronounced at higher frequencies due to the nonlinear characteristics of FET-based switched.